linux-kernel - Re: [PATCH v18 07/15] fs/namespace: untag user pointers in copy_mount

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Message-ID: <20190624175009.GM29120@arrakis.emea.arm.com>
Date:   Mon, 24 Jun 2019 18:50:09 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Andrey Konovalov <andreyknvl@...gle.com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, amd-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, linux-rdma@...r.kernel.org,
        linux-media@...r.kernel.org, kvm@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>,
        Yishai Hadas <yishaih@...lanox.com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Alexander Deucher <Alexander.Deucher@....com>,
        Christian Koenig <Christian.Koenig@....com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Jens Wiklander <jens.wiklander@...aro.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Leon Romanovsky <leon@...nel.org>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Dave Martin <Dave.Martin@....com>,
        Khalid Aziz <khalid.aziz@...cle.com>, enh <enh@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christoph Hellwig <hch@...radead.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>,
        Evgeniy Stepanov <eugenis@...gle.com>,
        Lee Smith <Lee.Smith@....com>,
        Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
        Jacob Bramley <Jacob.Bramley@....com>,
        Ruben Ayrapetyan <Ruben.Ayrapetyan@....com>,
        Robin Murphy <robin.murphy@....com>,
        Kevin Brodsky <kevin.brodsky@....com>,
        Szabolcs Nagy <Szabolcs.Nagy@....com>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v18 07/15] fs/namespace: untag user pointers in
 copy_mount_options

On Mon, Jun 24, 2019 at 04:32:52PM +0200, Andrey Konovalov wrote:
> This patch is a part of a series that extends kernel ABI to allow to pass
> tagged user pointers (with the top byte set to something else other than
> 0x00) as syscall arguments.
> 
> In copy_mount_options a user address is being subtracted from TASK_SIZE.
> If the address is lower than TASK_SIZE, the size is calculated to not
> allow the exact_copy_from_user() call to cross TASK_SIZE boundary.
> However if the address is tagged, then the size will be calculated
> incorrectly.
> 
> Untag the address before subtracting.
> 
> Reviewed-by: Khalid Aziz <khalid.aziz@...cle.com>
> Reviewed-by: Vincenzo Frascino <vincenzo.frascino@....com>
> Reviewed-by: Kees Cook <keescook@...omium.org>
> Reviewed-by: Catalin Marinas <catalin.marinas@....com>
> Signed-off-by: Andrey Konovalov <andreyknvl@...gle.com>
> ---
>  fs/namespace.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 7660c2749c96..ec78f7223917 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -2994,7 +2994,7 @@ void *copy_mount_options(const void __user * data)
>  	 * the remainder of the page.
>  	 */
>  	/* copy_from_user cannot cross TASK_SIZE ! */
> -	size = TASK_SIZE - (unsigned long)data;
> +	size = TASK_SIZE - (unsigned long)untagged_addr(data);
>  	if (size > PAGE_SIZE)
>  		size = PAGE_SIZE;

I think this patch needs an ack from Al Viro (cc'ed).

-- 
Catalin